Chapter 20 Medical suction apparatus
Main components
The main components of a medical suction system are:
The schematic drawings in Fig. 20.1 outline the methods used in suction apparatus, using internationally agreed symbols.
Energy source
Suction apparatus requires an energy source that generates a sub-atmospheric pressure. This is colloquially referred to as a vacuum source, despite the fact that a true (high) vacuum is rarely achieved. In fact, the pressure required is only a maximum of 60 kPa less than the normal environmental pressure (see Further reading).
Vacuum source
The sub-atmospheric pressure required may be generated by:
• an electric motor or other source of rotational energy that may be used to drive a mechanical pump, various forms of which are shown in Figs 20.2A–E.
• pneumatically driven pumps that usually work on the Venturi principle (Fig. 20.2F). The driving force may be air, oxygen, steam or water
• a manually (or foot) operated spring-loaded bellows arrangement with unidirectional valves (Fig. 20.2G).
Pump types
Fig. 20.2A shows a piston pump, which is capable of creating high vacuum but, in transportable models, has a relatively low displacement (i.e. can only sustain low flow rates). Fig. 20.2B shows a diaphragm pump, which is a variation of the piston pump. It is mechanically simpler but is also frequently much noisier. One reason for the increased noise is that often, rather than using a conventional rotating electric motor, a much simpler large electromagnet displaces the diaphragm. Fig. 20.2C shows a form of rotary pump that can produce a high vacuum without conventional one-way valves. Fig. 20.2D shows a rotary pump capable of producing very high flows, as would be required in a dental surgery. It works in the same way as a vacuum cleaner and has the disadvantage of being extremely noisy. Fig. 20.2E shows a pump that works on the principle of the Archimedean screw. This type of pump can produce a high vacuum for a comparatively small size of machine. Fig. 20.2F shows the principle of a Venturi pump which makes use of the Bernoulli effect. Compressed fluid (gas or liquid), passing through a narrow orifice, creates a region of negative pressure beyond that orifice, which can be used to entrain adjacent air/debris. The main disadvantage of this simple affair is that it uses and is thus wasteful of large volumes of driving fluid, which is usually oxygen from cylinders. However, it does have the virtue of being extremely portable (Fig. 20.3). Finally, Fig. 20.2G shows a simple bellows mechanism with a pair of one-way valves, as would be used in manually operated suction apparatus.
